1. Field of the Invention
This invention relates to a method for positioning a workpiece having at least one marking, in particular a wafer for highly integrated circuits, which is to be positioned in a charged-particle beam apparatus relative to a scanning field or a mask and in which a scanning beam scans the workpiece line by line and a signal generated by the scanning controls a writing beam which is caused to travel over the screen of a display apparatus.
2. Description of the Prior Art
In manufacturing highly integrated (LSI) circuits on a wafer, the separate areas which are to be differently doped, and the conductors connecting these areas, must be fabricated in several sequential process steps. Between each of these steps, the wafer surface is covered with photoresist varnish and the varnish film covering the wafer is exposed in selected areas to either light or electrons. The latter method (exposure to electrons) is particularly important if with progressing miniaturization of the integrated circuits, the resolution limit for light is exceeded. Regardless the method used to expose the photoresist layer on the wafer, however, the process must be repeated several times during the manufacturing process.
It is absolutely necessary to position the wafer each time one of the process steps is carried out to the same position with an accuracy which corresponds at least to the smallest spacing between the individual elements of the circuits, i.e., the capacitors, transistors, resistors, etc. of the circuits. Such positioning is achieved as follows utilizing a raster method:
The workpiece, for example, a wafer, to be aligned is scanned line by line (by either analog or digital methods) by a fine scanning beam, for example, a focused electron beam. In analog scanning, the deflection signal for deflecting the scanning beam is varied continuously. In digital scanning, the deflection signal is varied stepwise and has a staircase-shaped waveform. This type of deflection is used when the scanning is controlled, for example, by a computer. A writing beam is deflected on a display apparatus in synchronism with the scanning beam and its intensity is controlled by a signal generated by the scanning beam on the workpiece. This signal may be generated in response to the detection of backscattered electrons, secondary electrons, X-rays, or the specimen (workpiece) current. The scanning enables an image of the workpiece to be constructed point by point on the display apparatus. Alignment of the workpiece relative to markings applied to it on the display apparatus is achieved by using imaged markings or by computer-controlled comparison of the actual-value and reference-value positions.
A modified positioning method is known for use in an electron-beam projection apparatus. In this method, a mask having the desired structure is reduction imaged by electron-beam optics on the workpiece. See J. Vac. Sci. Technol., Vol. 12, No. 6 (1975), pages 1135 to 1140, and U.S. Pat. No. 3,876,883. To effect positioning, the illumination ray path is changed so that a fine electron beam is produced in the region of the mask and the workpiece. The signals emanating from the workpiece surface produce two superimposed images which are displayed on a display apparatus. One of these images is the sharp reproduction of the surface of the workpiece with alignment markings. The other image is a shadow image of the mask which is generated by suppressing electron rays in the mask plane by using the mask itself. The position of these two images enables the workpiece to be aligned relative to the mask.
In another known method, only an intensity profile along individual raster lines (line scan) is produced transversely through the markings on the workpiece and no complete image of the workpiece surface is recorded. This method is described in IEEE Transactions on Electron Devices, Vol EB-17, No. 6 (June, 1970), pages 450 through 457. The position of the marking signal relative to the raster field enables the deviation from a reference position, for example, the center of the raster field, to be determined. This can be achieved, for example, by comparison with markings on the display apparatus, or by counting the steps of the signal waveform generated when digital raster scanning is used. Using this positioning method, the alignment is achieved in only one direction at first. To complete alignment of the workpiece, the process must be repeated for the other directions. The position of a single point, for example, the maximum of the intensity distribution or the edge of a marking is almost only utilized to determine the deviation from the reference position. In the case of poorly made or damaged masks, this can cause considerable error. The latter error can be corrected, at least partially, by repeating the measurements at different points of the marking and then averaging these measurements. The disadvantage of this, however, is that it substantially increases the effort involved in making the measurement.